Note [Handling overloaded and rebindable patterns]

Overloaded patterns and rebindable patterns are desugared in the renamer
using the HsPatExpansion mechanism detailed in:
Note [Rebindable syntax and XXExprGhcRn]
The approach is similar to that of expressions, which is further detailed
in Note [Handling overloaded and rebindable constructs] in GHC.Rename.Expr.

Here are the patterns that are currently desugared in this way:

* ListPat (list patterns [p1,p2,p3])
  When (and only when) OverloadedLists is on, desugar to a view pattern:
    [p1, p2, p3]
  ==>
    toList -> [p1, p2, p3]
              ^^^^^^^^^^^^ built-in (non-overloaded) list pattern
  NB: the type checker and desugarer still see ListPat,
      but to them it always means the built-in list pattern.
  See Note [Desugaring overloaded list patterns] below for more details.

We expect to add to this list as we deal with more patterns via the expansion
mechanism.